The present disclosure relates to exhaust gas purification catalysts.
Exhaust gas purification catalysts for purification of hydrocarbon (HC), CO, and nitrogen oxide (NOx) in engine exhaust gas often contain Pt, Pd, Rh, etc., as a catalytic metal. For example, a catalyst which is close-coupled to an exhaust manifold may be heated to a temperature as high as around 1100° C. In this case, even when the catalytic metal is dispersed and supported on activated alumina particles having a large specific surface area, the catalytic metal agglomerates, and purification performance is gradually reduced. According to conventional technologies, an increased amount of the catalytic metal is supported on the activated alumina particles etc. in expectation of this agglomeration. However, the catalytic metal is rare metal, and increasing the amount of the supported catalytic metal is not preferable in view of conservation of resources.
As a solution to this problem, a catalyst in which the amount of the catalytic metal is reduced by doping a support, such as Ce-containing oxide etc., with a catalytic metal has been and is being put into practice. For example, a Rh-doped catalyst has been practically used, in which Rh is provided at lattice points or between the lattice points of CeZr-based mixed oxide (composite oxide), and Rh is partially exposed on the surfaces of the mixed oxide particles. The CeZr-based mixed oxide has originally been known as a support capable of storing and releasing oxygen. When doped with Rh, the CeZr-based mixed oxide can store a significant amount of oxygen, and can store and release oxygen at a significantly increased rate. Thus, even when the amount of the catalytic metal is reduced, the Rh-doped catalyst shows significant purification performance as a three-way catalyst used for purification of the exhaust gas which alternately shows rich and lean A/F ratios.
For example, Japanese Patent Application No. 2006-334490 describes catalytic powder in which CeZr-based mixed oxide doped with a catalytic metal is supported on the surface of CeZr-based mixed oxide containing no catalytic metal. The catalytic powder is obtained by mixing and dispersing the CeZr-based mixed oxide containing no catalytic metal in a solution containing Ce, Zr, and the catalytic metal, and adding ammonia water to the obtained solution to cause coprecipitation. Then, a slurry prepared by mixing the obtained catalytic powder, alumina, a zirconia binder, and water is applied to a honeycomb support, dried, and baked. In this way, the three-way catalyst is obtained.
Japanese Patent Application No. 2008-62130 describes a three-way catalyst which includes a catalyst layer containing CeZrNd mixed oxide having a number average particle diameter of 20-50 nm, CeZrNd mixed oxide having a number average particle diameter of 100-200 nm, and a zirconia binder. The degree of deterioration of the obtained three-way catalyst can be determined at high precision based on an oxygen concentration.
Japanese Patent Application No. 2007-31192 describes sol of ceria-zirconia solid solution used for a catalyst for purifying automobile exhaust gas, wherein particles of the sol have an average particle diameter of 5-100 nm. Japanese Patent Application No. 2002-239390 describes an HC storing catalyst in which an Ag zeolite layer and a Pd catalyst layer are stacked on a support. In this catalyst, hydrated alumina is used as a binder of the Ag zeolite layer, activated alumina is used as a support of the Pd catalyst layer, and a zirconia binder is used as a binder of the Pd catalyst layer, thereby preventing exfoliation of the Pd catalyst layer.